For example, PTL 1 discloses a storage system providing a virtual logical volume (hereinafter, VVOL) according to thin provisioning (also referred to as dynamic provisioning). This kind of storage system includes the VVOL and a pool constituted by multiple pool volumes (hereinafter, pool VOLs). This kind of storage system provides the VVOL to a host computer. The size of VVOL provided to the host computer may be equal to or more than the size of the pool.
The VVOL includes multiple virtual pages (virtual storage regions). The pool VOL includes two or more real pages (actual storage regions) which are cut out from one or more storage devices (for example, RAID (Redundant Array of Independent (or Inexpensive) Disks) group).
When there is a virtual page that is not allocated to a virtual page that belongs to write destination (for example, address) designated with a write command by the host computer (virtual page for which no real page is allocated from the pool), the storage system allocates a real page to the virtual page from the pool, and writes data according to the write command to the allocated real page.
PTL 1 also discloses a technique for writing predetermined data such as zero (hereinafter, zero data) to the entire region of the real page allocated to the virtual page storing invalid data (data that cannot be used) and recovering the real page storing only zero data (real capacity) as the allocatable real capacity to the pool (hereinafter, recovery of real capacity).